In this application, I propose a project of technology development, validation, and translation aimed at providing a better imaging marker of axonal damage in patients with multiple sclerosis (MS). This award will provide protected research time and training in preparation for independent R01 funding. Multiple sclerosis is the leading non-traumatic cause of disability in young adults. Axonal damage is considered the pathologic substrate underlying clinical disability in MS but currently lacks a reliable imaging biomarker. Advanced diffusion MRI techniques with microstructural specificity for axon diameter and density offer the potential to improve the understanding of axonal damage in MS. The goal of the proposed research is to develop, validate and translate a new diffusion MRI method called Tract Caliber MRI to provide more direct and accurate measures of axonal damage in people with MS than existing MRI techniques. I will optimize the Tract Caliber technique to measure axon diameter and density within lesions and normal-appearing white matter in people with MS compared to healthy controls. The Tract Caliber measurements will then be validated against axon diameter and density measurements from histopathology performed on ex vivo brain tissue obtained at autopsy from MS patients. Once validated, Tract Caliber measurements of axon diameter and density in the corpus callosum and corticospinal tracts will be correlated with physical disability and cognitive dysfunction in MS patients compared to conventional MRI, magnetization transfer imaging, and diffusion tensor imaging in order to address the clinical utility of these different imaging techniques in tracking disease progression. This research proposal will enable me to achieve my short-term goal of becoming an independent investigator in translational neuroimaging. My ultimate goal is to lead an independently-funded research program in the development and clinical translation of novel imaging biomarkers for improving the care of patients with neurological diseases such as MS. To facilitate my progression to a position of independence, I have designed with my mentors a career development plan involving tailored didactic coursework, workshops, seminars, and hands-on training that will provide me with formal education in histopathological validation, disease pathogenesis, and clinical evaluation of patients with MS. The primary mentor for this project is Dr. Lawrence Wald, an expert in developing advanced diffusion MRI technology to solve specific biological and clinical problems. Dr. Howard Weiner will serve as the secondary mentor, lending his renowned expertise in MS. In addition, I will work with a team of collaborators/consultants from MGH (Dr. Bruce Fischl, Dr. Eric Klawiter), Boston University (Dr. Kathleen Rockland), and the Cleveland Clinic (Dr. Bruce Trapp), who will provide additional expertise and guidance as part of my scientific advisory committee. This research will be performed at the Martinos Center for Biomedical Imaging, a world-leading institution for translational neuroimaging and the study of the brain in health and disease. The project will greatly benefit from the unique resources of the Martinos Center, including the one-of-a-kind 3 Tesla Human Connectome MRI scanner equipped with ultra-high gradient strengths ideal for probing axonal microstructure. We will recruit patients with MS from the large patient population served by the MGH Multiple Sclerosis Clinic and Partners MS Center for the development and translational aims of this project and obtain fixed brain tissue samples from the MS tissue bank established at the Cleveland Clinic for the validation aim. This collaborative framework will help me foster relationships with leading experts in MS and will increase the impact and visibility of my research.